The present invention relates generally to high speed packet data wireless communication systems, and in particular to reducing soft virtual handoff latency by transmitting data from a predicted queue offset at a target cell.
A recent development in wireless communication systems is the advent of high-rate packet data (HRPD) systems, in which packet data for multiple Access Terminals (AT) is broadcast by the Access Network (AN) on one or more shared, high-bandwidth, forward traffic channels. As with any wireless communication system, mobility management in HRPD systems requires the ability to transfer or “hand off” a mobile AT from a current, or source, serving sector to a new, or target, serving sector, as the AT moves further from the radio transceivers of the source sector. In the HRPD system the mobile is served by only one sector in the forward link at any one given point in time. The handoff may be between serving sectors of different cells, known as soft virtual handoff or cell selection, or it may be between serving sectors of the same cell, known as softer virtual handoff. The present invention is directed to soft virtual handoff; accordingly, reference is made herein to the source and target cells rather than sectors.
In HRPD systems, an AT selects a serving cell by covering its reverse link Data Rate Control (DRC) signals with the Walsh code of the target cell. In CDMA 1×EV-DO Revision A, the Data Source Control (DSC) reverse link channel was added to communicate an AT's intention to switch serving cells prior to actually doing so by changing its DRC cover. This provides the Access Network time to establish a network dialogue to the target cell, thus reducing delays in the handoff that cause noticeable service interruption to users for delay-sensitive services.
The DSC signal indicating a target cell allows the Access Network to establish a new network dialogue and begin transmitting data packets to the target cell, which queues the data prior to its selection by the AT's DRC cover. However, until the AT actually selects the target cell, data for the AT continues to be transmitted by the source cell. Thus, the target cell, when selected by the AT, will likely transmit some data packets that the AT has already received from the source cell, resulting in a waste of air interface resources and incurring a delay at the AT until the target cell advances to new packets (i.e., those not already transmitted by the source cell). Alternatively, following the soft virtual handoff, the source cell may send to the target cell, along a side haul connection, queue pointers identifying exactly which packets have been transmitted. In this case, the AT must wait until the target cell has received the queue pointers, before the target cell can begin transmitting data. In either case, the AT experiences a delay due to queue synchronization between the source and target cell. Queue synchronization is not an issue in the case of softer virtual handoff.